1. Field of the Invention
The present invention relates to a correlation type velocity detecting apparatus capable of sensing noise signals obtained at two points spaced from each other by a certain fixed distance in the direction of motion of the object, and of detecting the velocity of the moving object according to the correlation between the two signals thus sensed.
2. Description of the Prior Art
FIG. 1 is a block diagram of a known flow meter used to detect the velocity of a fluid through a duct according to the correlation thereof. In this diagram, a fluid or object to be measured flows through a duct 1. Ultrasonic transmitters 21 and 22 are attached to the duct 1, and ultrasonic receivers 31 and 32 are attached to the duct 1 across from the ultrasonic transmitters 21 and 22 respectively. An oscillator 4 applies a drive signal to the ultrasonic transmitters 21 and 22. Demodulators 51 and 52 demodulate the signals from the ultrasonic receivers 31 and 32 and apply them to a correlator 6. The correlator 6 takes the cross correlation of input signals received from the demodulators 51 and 52.
Such a conventional flow meter operates as follows. When the ultrasonic transmitters 21 and 22 are driven by the signal from the oscillator 4, ultrasonic signals produced therefrom propagate through the fluid to be measured and reach the ultrasonic receivers 31 and 32. In propagation paths P.sub.1 and P.sub.2, the ultrasonic signals are affected by the flow noise such as inherent fluctuations or eddies existing in the fluid, so the signals obtained from the ultrasonic receivers 31 and 32 are modulated by the flow noise. (Here, the term "modulation" is defined to include frequency modulation, amplitude modulation, phase modulation and so forth.) The demodulators 51 and 52 serve to demodulate the modulated signals from the ultrasonic receivers 31 and 32. If the pattern of the noise signal in the flow remains unchanged between the two receivers, a signal x(t) as shown in FIG. 2(a) is obtained from the demodulator 51, and a signal y(t) as shown in FIG. 2(b) is obtained from the demodulator 52. The propagation paths P.sub.1 and P.sub.2 of the ultrasonic signals are spaced apart from each other by a distance L in the direction of flow of the fluid to be measured, so the fluid having passed through the propagation path P.sub.1 comes to pass through the path P.sub.2 after the lapse of time .tau..sub.0. Accordingly, the signals x(t) and y(t) obtained from the demodulators 51 and 52 are represented by the following equation. EQU y(t)=x(t-.tau..sub.0) (1)
The flow velocity v of the fluid to be measured, the time .tau..sub.0 and the distance L between the propagation paths P.sub.1 and P.sub.2 are related as follows: EQU .tau..sub.0 =L/v (2)
The correlator 6 takes the cross correlation between signals x(t) and y(t) from the demodulators 51 and 52, and produces a correlation signal .phi..sub.xy of FIG. 3(a) through the output terminal thereof. The cross correlation signal .phi..sub.xy can be expressed as ##EQU1## Equation (3) represents the autocorrelation of x(t), which is maximum when .tau.-(L/v)=0. That is, .phi..sub.xy has a maximum value when .tau.=(L/v)=.tau..sub.0. Therefore, by obtaining .tau..sub.0 at which .phi..sub.xy becomes maximum in FIG. 3, the flow velocity v of the fluid can be found from Equation (2) as follows: EQU v=(L/.tau..sub.0) (4)
where L is the distance between the propagation paths P.sub.1, P.sub.2 and remains constant.
Since the flow meter based on the above principle utilizes a signal inherent to the turbulent flow, it has an advantage of not requiring a flow-rate measuring means such as an orifice or eddy generating element in the fluid. However, the measurement may be rendered impossible by the periodicity of the correlation function resulting from the periodic disturbance such as vibration of the duct 1 or internal pressure variation in the fluid. That is, where the ultrasonic receivers 31 and 32 pick up the inphase disturbance component z(t) as a result of the vibration of the duct 1 or the internal pressure variation in the fluid, the cross correlation signal .phi..sub.xy ' is expressed as follows: ##EQU2##
In Equation (5), the final term within the integral represents the autocorrelation of the disturbance component z(t). As this value increases, .phi..sub.xy becomes unobtainable rendering measurement impossible. (Refer to FIG. 3(b))
An object of the present invention is to provide an improved relative-velocity detecting apparatus which is free from influence of any periodic disturbance component such as the vibration of an object or a duct to which a detecting means is attached or the pressure variation in a moving fluid.